1. Field of the Invention
The present invention relates to the acquisition of images by optical tomography, whereby it is possible to obtain images formed by light ray intensities originating from an object to be studied, these intensities being a function of the depth in that object. These optical intensities can be obtained either by the reflection of rays on or in the object, or by the transmission of light through said object.
More particularly, the invention concerns the area of optical low-coherence interferometry and applies the principle of the Michelson interferometer.
FIG. 1 in the appended drawings represents such an interferometer. It includes a light source S with a broad spectral bandwidth which consequently has a low coherence length. The beam from this source is steered to a beam splitter SF which splits the beam from the source into a beam lighting an object O to be studied and a beam striking a reference mirror M.
In this figure, the beam striking the object O and the beam striking the mirror M are respectively reflected and steered through the beam splitter SF to be recombined and illuminate a photodetector PC. These reflected beams interfere with each other constructively and destructively, forming an interference fringe only if the difference of the optical distances covered is less than the coherence length of the source.
This interferometric device can therefore be used to obtain an indication, for example, as to the nature of the surface of the object. However, in the form that has just been described, this interferometer cannot be used to obtain tomographic information on the object, in other words, information obtained by reflection from several points inside the object, located by depth.
2. Description of the Prior Art
To obtain such depth information, depth scanning is already an accepted practice (see, for example, the paper by E. A. Swanson et al. in OPTICS LETTERS/Vol. 18, No. 21/Nov. 1, 1993). In this case, the interferometer is implemented using optical fibers and couplers, which does not fundamentally affect the measurement principle. However, to obtain information from different depths of the object, successive measurements are taken, each time changing the position of the reference mirror to modify the length of the optical path in the arm of the device containing said mirror (hereinafter called the reference arm). In FIG. 1, this movement is symbolized by arrow B.
The result of this is an interference graph as shown in FIG. 2 when the object O corresponds to an interface in which the light intensity I striking the photodetector PC is given as ordinates and the longitudinal position of the reference mirror M is given as abscissae (by convention, known as the Z-axis position which also reflects the depth position of the point of the object having given rise to the interference fringe concerned). It should be noted that the resolution of the measurement depends on the coherence length of the source S indicated by arrow Lc.
Such a scanning-based measurement process involving several measurements spaced out over time presents certain disadvantages, because, in addition to the fact that the measurement is necessarily fairly lengthy, it is seriously disrupted if the object is subject to movements. This may be the case, for example, in the medical domain, which has emerged as a particularly promising application, and in particular, when measurements are made on certain parts of the eye, such as the cornea or the retina. Furthermore, to move the mirror, it is necessary to use a mechanical movement element, which can result in vibrations and, possibly, a drop in performance over time.
Another disadvantage of this process is that the measurement applies only to points that are aligned with each other along an axis extending depthwise and defining the direction of the light beam reflected by the object (depth reflection profile).
Thus, to obtain the image of a slice taken depthwise through the object, various series of measurements must be performed successively as described above, that can be qualified as one-dimensional but are offset laterally from each other, to obtain groups of intensity values that must then be processed to convert these series of one-dimensional measurements into a two-dimensional result representative of the profile of a slice of the object. Clearly, this procedure exacerbates the disadvantages of the one-dimensional measurement in terms of measurement time and susceptibility to movements of the object.
The object of the invention is to provide a measurement device of the type briefly described above, which can be used to obtain instantaneously the entire result of the measurement, applied to a depth alignment of points, on a depth slice through the object, even a three-dimensional portion of the object.